Among other factors, and not meaning to limit anything...
Congress has declared me, a 39-yo able-bodied male not in the National Guard, a member of the US militia. Per the Selective Service System, I can be called up for service. Being 39 and not 19, should I be called up for any reason (insert scenario here), I have reason to believe there would be only poor equipment available and little time to train. The government, at various levels, has decided to entirely neglect training or equipping me during peacetime, and would likely be unable to under duress. As I obviously have a very personal interest in this, and the Founding Fathers did too, both in 1777 and in 2007 there is clearly an individual right for me to obtain (at minimum) standard modern military weaponry and training to match.
Any attempt to deny the individual right to modern military arms, starting with the M4, harms both myself (by hindering my ability to perform in mortal combat) and my nation (by hindering the ability of the aggregate militia to effectively fight).
As for discussions of weapons inconceivable in 1777: let's start with the M4 - it (or the AK-74, and assorted relatives of both) are simply the modern standard not outrageously different from early rifles. Next consider other weapons not having orders of magnitude more orders of magnitude more power ... .50BMGs, RPGs, and artillery are not mind-blowingly far beyond what our Founding Fathers personally owned. When we can personally buy those with little or difficulty, then we can consider moving the discussion into the WMD realm; few who go there early in the discussion have any interest in securing a reasonable modern interpretation of RKBA.
Wake me up when I can buy a select-fire M4.